Rapid prototype injection molding

ABSTRACT

Disclosed is a method and apparatus for making a prototype injection molded part. An extruder of the type used for fused deposition modeling injects production thermoplastic material into a heated nonconductive plastic mold tool slowly at low pressure in an isothermic process. The mold tool may be built from a CAD drawing by fused deposition modeling or another rapid prototyping technique. Using the present invention, an injection molded prototype part can be made from a digital representation of the part within 24 hours by an engineer in an office environment.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a United States national phase entry of PCTInternational Application No. PCT/US03/11854, filed on Apr. 17, 2003,which claims the benefit of U.S. Provisional Application No. 60/373,332,filed Apr. 17, 2002.

BACKGROUND OF THE INVENTION

The present invention relates to prototyping of injection moldedobjects, and more particularly to an office-compatible method forrapidly making plastic injection molded prototypes parts.

In a typical injection molding process, plastic is injected at highpressures, extremely quickly, into a thermally conductive metal mold.The molded part is quickly cooled to a temperature at which it can beremoved from the mold. The part is then quickly ejected from the mold sothat another part can be made, and so that the part does not becomestuck on the mold (due to shrink differential). Cooling of large partscontinues on a fixture. The goals of production injection modeling areto produce a high quantity of high-quality parts in a short turn-aroundtime. A thirty second cycle time or less for the making of each moldedpart is typical.

In order to produce a three-dimensional object in a typical injectionmolding process, it is necessary to prepare a mold tool that has acavity which is complementary to the desired shape of thethree-dimensional object. The mold tool generally consists of twoopposing halves, which mate together to define the mold cavity. The moldtool is normally machined out of steel or other metal which is capableof withstanding high temperature and pressure when hot liquid isinjected into the mold. In use, the mold tool is inserted into a frameof an injection molding machine, and held in place with high clampingforces to oppose pressure generated inside the mold. The time and skillrequired to prepare the mold tool are both significant. The machiningmust be done by skilled craftsmen, and includes the incorporation of asprue through which the molding material is injected, a vent, coolinglines and ejector pins. Typically, this process involves placing anorder with an outside vendor and waiting several weeks or months fordelivery, at high cost.

Before undergoing the expense and long lead time associated withconventional metal mold manufacturing, it is desirable to produce aprototype of the part that will have similar characteristics to theproduction part. The goal is produce a prototype having characteristicssufficiently close to that of the desired final manufactured part so asto permit a close prediction of part performance. Various additiveprocess rapid prototyping (RP) technologies are commonly used to makeprototype parts in the design stages of a part. These rapid prototypingtechnologies include fused deposition modeling (FDM), stereolithography(SLA), selective laser sintering (SLS), laminated object manufacturing(LOM) and jet technology. These additive process techniques produceprototypes useful for evaluating the fit, form and function of a partdesign, to gain preliminary part approval and to accelerate productdevelopment. The strength of a final production part is not, however,replicated in prototypes created by these rapid prototyping techniques.The additive processes create layers, layered stress points and voids inthe part resulting in a different internal stress structure than that ofthe homogeneous injection-molded part. Additionally, many materials usedin these processes are weak.

Various methods have been developed for creating mold tools used to makeprototype injection molded parts, which may be referred to as “bridgetooling” or “temporary tooling.” A number of these methods utilize rapidprototyping techniques, particularly, stereolithography. For example,U.S. Pat. No. 5,439,622 describes the use of stereolithography to form amold shell, which is then reinforced with an incompressible material andcoated with a thermally conductive material. U.S. Pat. No. 5,989,679describes a mold tool formed by injecting a strengthening material intocavities within an object formed by stereolithography. U.S. Pat. No.5,952,018 describes a mold tool, including an ejection valve within themold tool, formed by stereolithography. U.S. Pat. No. 5,641,448describes the making of a mold tool by depositing a metal coating onto aplastic mold shell produced by stereolithography.

The use of rapid prototyping to create molds for use in processes otherthan injection molding are also known. For example, U.S. Pat. No.6,073,056 describes a mold built by stereolithography or fuseddeposition modeling used to form a vacuum cast part. U.S. Pat. No.6,103,156 describes the making of a prototype part by pouring athermoset into a mold formed by a rapid prototyping technique.

Techniques are also known which use a part formed a rapid prototypingprocess as a master mold pattern to create a prototype mold tool. Forexample, U.S. Pat. No. 5,189,781 describes the use of a prototype partas the pattern for making a sprayed metal mold. U.S. Pat. No. 5,707,578uses a prototype created by stereolithography as a master mold.

A commercial process known as the Swiftool™ process uses a prototypepart, which may be made by a rapid prototyping technique, as a patternfor creating an epoxy mold. The process takes several days. Anothercommercial process known as 3D Keltool® makes bridge tooling in a periodof several days in a metal-powder sintering process, starting from amaster pattern made by stereolithography. Yet another commercial systemcalled AIM™ builds mold tools by stereolithography using UV-sensitivematerials.

There is a need for a more rapid, easy to use and low cost method ofcreating a small number of prototype injection molded parts, that iscompatible with an office environment.

BRIEF SUMMARY OF THE INVENTION

The present invention is a method and apparatus for making a prototypeplastic injection molded part using a non-conductive plastic mold toolthat may be built using a rapid prototyping technique. The prototypepart is preferably made from a production thermoplastic material whichallows an assessment of the strength of a production part. Theproduction thermoplastic is injected into a mold cavity of the mold toolby an extruder in a slow, low pressure process, under isothermicconditions to form the prototype part. The extruder may be a filamentpump (i.e., an FDM head), a piston pump, a screw pump, or otherextruder. The prototype part is cooled in the mold cavity toapproximately room temperature. The production thermoplastic and theplastic that forms the mold tool are selected so as to haveapproximately the same shrink characteristics upon cooling. A fuseddeposition rapid prototyping machine may be the apparatus used toperform the injection.

In a preferred embodiment, the mold tool is made by a fused depositionmodeling technique. In one embodiment, the mold tool is built in two ormore portions, wherein layers of thermally solidifiable non-conductivematerial are deposited in a predetermined pattern according to computerfile data representing the mold shape. Each mold portion includes a moldsurface, a mating surface, and a base which supports the mold and matingsurfaces. Together the mold portions define the mold cavity. In analternate embodiment, the mold tool is made from a soluble material andhas a single-piece construction.

Using the present invention, an injection molded prototype part can bemade from a digital representation of the part within 24 hours by anengineer in an office environment. The design engineer is empowered tobuild injection modeled parts from CAD files, similar to how rapidprototyping empowered the design engineer to build prototypes from CADfiles.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a top plan view of two mold portions of an exemplary mold toolfor use with the present invention produced by fused deposition modeling

FIG. 2 is a sectional view of the two mold portions of FIG. 1, takenalong a line 2—2 of FIG. 1 and mated together to define a mold cavity.

FIG. 3 is a schematic illustration, in partial section, of a rapidprototyping injection molding machine according to the presentinvention.

FIG. 4 is a flow diagram of the process of making a prototype injectionmolded part in accordance with the present invention, using a mold toolmade by fused deposition modeling.

DETAILED DESCRIPTION

The method and apparatus of the present invention builds a prototypeinjection molded part using a non-conductive plastic mold tool, which ispreferably built using a rapid prototyping technique. The injection isdone by extruding a liquified ribbon of material from an extruder of thetype used in a fused deposition modeling machine. Extruded material isinjected into a mold cavity of the mold tool at a low speed and lowpressure. After the mold cavity is filled, the prototype part thuscreated is allowed to cool inside of the mold tool. The prototype partis preferably made from a production thermoplastic material which allowsan assessment of the strength of a production part. A fused depositionmodeling machine may be used for the prototype injection moldingapparatus.

In contrast to manufacturing injection molding processes, the presentinvention uses low pressures, low flow rates and slow cycle times,allowing the use of a non-reinforced plastic mold tool and providing asafe process compatible with office use. Using the present invention, aninjection-molded prototype can be formed from a digital representationof the desired prototype part within about a 24-hour time period, by anengineer in an office-friendly environment.

FIG. 1 shows two halves of an exemplary mold tool 10 for use in carryingout the present invention. The exemplary mold tool is the subject ofInternational Application No. PCT/US03/10219 entitled “LayeredDeposition Bridge Tooling,” filed Apr. 4, 2003, and assigned to the sameassignee as the present application, which is hereby incorporated byreference as if set forth fully herein. A first portion 12 of mold tool10 includes a recessed mold surface 14 corresponding to the shape of afirst half of a desired prototype molded part. A second portion 16 ofmold tool 10 includes a recessed mold surface 18 corresponding to theshape of a second half of the desired prototype molded part. The moldportions 12 and 16 each have a mating surface 17 and a base 20 shown inFIG. 2, which supports the mold surfaces 14 and 18 and the matingsurfaces 17. When the mating surfaces 17 of the mold portions 12 and 16are mated together as shown in FIG. 2, the mold surfaces 14 and 18define a mold cavity 19, which has the shape of the desired prototypepart. For prototype molded parts that have interior cavities, the moldtool 10 further comprises a mold core.

The mold portions 12 and 16 each also include a sprue channel 22, a ventchannel 24, and four alignment holes 26. The sprue channels 22 allow forthe placement of a sprue which will be inserted in a final assembly ofthe mold tool 10, providing a path for the injection of molten plasticinto the mold cavity 19. The vent channels 24 together form a passagefor the venting of gas from the mold cavity 19 when the mold tool 10 isassembled.

The alignment holes 26 receive screws or pins, which align and holdtogether the mold tool portions 12 and 16 in assembly of the mold tool10. The mold tool 10 may also optionally include cooling lines forintroducing a flow of coolant during an injection process.

In an alternate embodiment, a mold tool is made from a soluble modelingmaterial and has a single-piece construction. The soluble materialpermits a single-piece construction, as the mold tool may be dissolvedfrom a prototype part after the part is formed. In contrast, a mold toolmade from an insoluble material is removed from a prototype part bymechanically disengaging the mold portions. A suitable soluble modelingmaterial is an alkali-soluble material comprising a base polymercontaining a carboxylic acid, and a plasticizer. The base polymercomprises a first comonomer (which contains carboxylic acid) and asecond comonomer that is polymerized with the first comonomer to providethermal and toughness properties suitable for fused deposition modeling.A preferred base polymer is comprised of methacrylic acid as the firstcomonomer and an alkyl methacrylate (e.g., methyl, ethyl, propyl orbutyl methacrylate, and combinations thereof), preferably methylmethacrylate, as the second comonomer. A desirable amount of theacid-containing first comonomer is 15–60 weight percent of the basepolymer. The base polymer is plasticized to attain rheologicalproperties desired for the modeling process. Most preferably, thealkali-soluble thermoplastic material contains between about 84 weightpercent and 74 weight percent of the base polymer and contains betweenabout 16 weight percent and 26 weight percent of the plasticizer, andhas a melt flow index of between about 5 g/10 minutes and 10 g/10minutes under a load of 1.2 kg at 230° C. A mold tool made from thealkali-soluble material is removed from the prototype part by placingthe mold tool containing the part in an alkaline bath. Thealkali-soluble modeling material is the subject of co-pending U.S.patent application Ser. No. 10/019,160 and corresponding InternationalApplication No. PCT/US00/10592 (published as WO 00/62994), which ishereby incorporated by reference as if set forth fully herein.

The mold tool for use in the present invention is preferably built by arapid prototyping process, such as by fused deposition modeling, fromcomputer file data representing the mold tool. The computer file data isderived from information available on the desired prototype molded part.For example, typically, the part is designed using a computer-aideddesign (CAD) system, and corresponding information relating to theoutline of the part is derivable from a CAD file defining the desiredpart. A computer program designs the mold portions in accordance withthe outline of the desired part, as the inverse of the desired partshape. For instance, software available from Moldflow Corporation, willdesign the mold portions in this manner. A further software program“slices” the computer representation of the mold portions intohorizontal layers.

Fused deposition modeling builds up three-dimensional objects, such asthe mold tool of the present invention, in layers by extruding moltenmodeling material in a predetermined pattern according to the computerfile data representing the mold tool. The modeling machine extrudes theroads of modeling material layer-by-layer, with each extruded roadhaving a thickness equal to the height of a slice. The extruded materialfuses to previously deposited material and solidifies upon a drop intemperature to form the mold portions. The mold portions may be builtsimultaneously in the modeling machine, or one at a time. In a preferredembodiment, the mold portions 12 and 16 are built from apolyphenylsulfone resin on a Stratasys® Titan™ FDM® fused depositionmodeling machine.

The sprue channels 22, the vent channels 24 and the alignment holes 26are preferably formed into the mold portions 12 and 16 as they arebuilt. This can be done by including such features in the computer filedata representing the mold tool 10. Alternatively, a sprue channel, ventchannel and/or alignment holes may be machined into the mold portions 12and 16 after they are built. The channels 22 and 24 and the alignmentholes 26 shown in the exemplary mold tool 10 are merely one example ofthe placement and design of such features. Alternative designs includevertical orientation of the channels 22 and 24, and forming a singlesprue channel or vent channel within one or the other of mold portions12 and 16.

The need for a vent channel in the mold tool 10 may be avoided bycontrolling the extrusion pattern of the roads so that the mold tool 10has an inherent porosity providing an open-cell matrix sufficient tovent gas from the mold cavity 19. Controlled-porosity fused depositionmodeling is taught in U.S. Pat. No. 5,653,925.

The exemplary mold tool 10 is formed from a non-conductive thermoplasticmaterial that will sustain the temperature and pressure of the injectionmolding process, so as to produce at least one prototype plasticinjection molded part. An exemplary thermoplastic comprises at least 50weight percent of a thermoplastic selected from the group consisting ofpolycarbonate, polystyrene, acrylics, amorphous polyamides, polyesters,polyphenylsulfone, polysulfone, polyphenylene ether, nylon, PEEK, PEAK,poly(2-ethyl-2-oxazoline), and blends thereof. The thermoplastic resinmay contain various fillers, additives and the like, as will beunderstood by those skilled in the art. A particularly preferredthermoplastic for use in creating a mold tool by fused depositionmodeling is a polyphenylsulfone-based resin.

FIG. 3 shows an exemplary rapid prototype injection molding apparatus 30in accordance with the present invention, in the process of making aprototype part. The apparatus 30 comprises an extrusion head 32 having adispensing tip 34, a material supply 36, a controller 38, a modelingenvelope 39, and a hollow sprue 35. The mold tool 10 is assembled andpositioned in the apparatus 30. Prior to assembly of the mold tool 10,the mold surfaces 14 and 18 are created with a release agent thatfacilitates removal of a completed part from the mold tool 10. Suitablerelease agents include dry film lubricants, and others that will berecognized by those skilled in the art. The mold tool 10 in mounted inthe modeling envelope 39. Sprue 35 is placed in the sprue channels 22,such the a dispensing end of the sprue 35 is directed into the moldcavity 19. The sprue 35 has an entry 37 at a top end thereof, designedto mate with the downward-facing extrusion head tip 34. The sprue entry37 receives and attached to the extrusion head tip 34, thereby providinga flow path from the extrusion head 32 into the mold cavity 19.Preferably, an insulator is provided for the extrusion head tip 34, sothat the tip 34 will not cause melting of the prototype part as it isbeing formed. Also, a pressure transducer (not shown) is placed in thesprue to monitor pressure in the mold cavity so that a predeterminedpressure may be maintained.

The apparatus 30 may be a fused deposition modeling machine. It shouldbe understood, however, that unlike fused deposition modeling, theprocess of the present invention involves no translational movement ofthe extrusion head. The extrusion head 32 may be of any type whichreceives a thermoplastic material and dispenses the material in a moltenstate through a dispensing tip at low flow rates and low pressure.Suitable extrusion heads have been developed for fused depositionmodeling, and include a liquifier pump, a piston pump and a screw pump.Each of these extrusion heads developed for three-dimensional modelingreceives a feedstock of thermoplastic in solid form, and heats thethermoplastic material to a desired temperature for extrusion.

In the exemplary embodiment, the extrusion head 32 receives a supply ofproduction thermoplastic material for creating the molded prototype partfrom the material supply 36, at a rate controlled by the controller 38.Where the extrusion head 32 of the exemplary embodiment is a liquifierpump, the material supply 36 comprises spooled flexible filament and theextrusion head 32 carries a set of feed rollers for advancing thefilament into the extrusion head at the controlled rate. Liquifier pumpsare disclosed, for example, in U.S. Pat. No. 6,004,124. Where theextrusion head 32 of the exemplary embodiment is a piston pump, thematerial supply 36 comprises cylindrical feed rods of thermoplasticmaterial fed in a batch process. A piston pump extrusion head isdisclosed in U.S. Pat. No. 6,067,480. Where the extrusion head 32 of theexemplary embodiment is a screw pump, the material supply 36 comprisespellets of thermoplastic material. A screw pump extrusion head isdisclosed, for example, in U.S. Pat. No. 5,312,224. Two-stage extrusionheads are also known in the art, and can also be used in practice of thepresent invention. A extrusion pump is disclosed in U.S. Pat. No.5,764,521, wherein the feedstock received from material supply 36 ispressurized in a two-stage process which may take various forms.

The extrusion head 32 may include an ultrasonic vibrator for creating athixotropic flow at the dispensing tip exit, such as is disclosed inU.S. Pat. No. 5,121,329. The ultrasonic energy would reduce theinjection pressure while increasing the flow rate of the productionthermoplastic.

For the production of a prototype part, production thermoplastic isprovided from the material supply 36 to the extrusion head 32, whichheats the production thermoplastic to an extrusion temperature anddispenses molten extruded material 33 through sprue 35 and into the moldcavity 19. Production thermoplastics that may be used in the presentinvention include, without limitation, ABS, polycarbonate, polystyrene,acrylics, amorphous polyamides, polyesters, polyphenylsulfone,polyphenylene ether, nylon, PEEK, PEAK, and blends thereof. Theproduction thermoplastic may, of course, include various fillers,additives and the like. Shrink characteristics of the mold tool plasticare matched to the shrink characteristics of the productionthermoplastic, which can be achieved by using amorphous thermoplastics.Also, the production thermoplastic must have a heat deflectiontemperature lower than a heat deflection temperature of the plasticwhich forms the mold tool, so that the mold tool will maintain itsshape.

FIG. 4 shows a flow diagram which summarizes an exemplary method ofproducing a prototype injection molded part in accordance with thepresent invention, using a mold tool made by fused deposition modeling.A CAD tool is used to generate computer file data representing a moldtool, in a step 40. The data is provided to a fused deposition modelingmachine, in a step 42. The mold tool is built in the fused depositionmodeling machine, in layers defined by the computer file data, in a step44. In a step 46, the mold surfaces and/or mating surfaces of the moldtool are smoothed to remove ridges unintentionally created in theformation of the mold tool.

In a preferred embodiment, the smoothing is done in a vapor smoothingprocess, which is the subject of International Application No.PCT/US03/10220 entitled “Smoothing Method For Layered DepositionModeling,” filed Apr. 4, 2003, assigned to the same assignee as thepresent application, and hereby incorporated by reference as if setforth fully herein. As is disclosed in said co-pending application, thesurfaces of the mold tool can be smoothed by placing the mold tool in avaporizer and exposing it to vapors of a solvent until a desired surfacefinish is obtained. The solvent is selected to be compatible with thematerial which forms the mold tool. Suitable solvents will react withthe material so as to soften and flow the material at the objectsurfaces. A preferred solvent for use with a wide range of amorphousthermoplastics is methylene chloride. Other suitable solvents will berecognized by those skilled in the art, for instance, an n-Propylbromide solution (e.g., Abzol®), perchloroethylene, trichloroethylene,and a hydrofluorocarbon fluid sold under the name Vertrel®. Vaporsmoothing will also serve to seal the surfaces of the mold tool. As istaught in said co-pending application, certain mold features may beidentified for solvent masking or for pre-distortion prior to the vaporsmoothing step, and the computer file data representing the mold toolmay include data identifying said features. Alternatively, smoothing canbe done by applying a liquid solvent. Other alternative smoothingtechniques include sanding, grinding, and thermal ironing.

The mold surfaces of the mold tool are then coated with a release agent,in a step 48 (a soluble mold tool may not need a release agent).Suitable release agents include dry film lubricants, and others thatwill be recognized by those skilled in the art. If needed, sprue andvent channels and alignment holes are machined into the mold tool priorto step 48. The mold tool is assembled in an prototype injection moldingapparatus according to the present invention, without the addition ofany conductive fill material or layers, in a step 50. The fuseddeposition modeling machine used to build the mold tool may be used alsoas the prototype injection molding apparatus.

In step 50, the sprue is positioned in the mold tool and attached to adispensing tip of an extrusion head. The mold may be clamped to afixture to hold it in place, using a clamp or other means. A clampingforce of less than or equal to about 10 tons will ensure officecompatibility of the injection process. The mold tool is then heated toa predetermined temperature, in a step 52.

Injection molding is then performed, in a step 54. Productionthermoplastic is injected from an extrusion head of the moldingapparatus into the mold tool, at low speed and low pressure. If anultrasonic vibrator is used in the extrusion head, a thixotropic flow ofthermoplastic will be injected. During the injection, pressure ispreferably monitored, such as by a pressure transducer placed in themold cavity or sprue. A system controller can adjust the extrusion flowrate based upon pressure reading from the transducer, to maintain thepressure within a target range. The target pressure will typically beless than 5000 psi and may be set at less than 500 psi, to as low asless than 20 psi (near-zero pressure).

Filling the mold cavity will typically take about 1–2 hours. During thistime, the temperature of the mold and the production thermoplasticremain approximately constant, as the plastic mold is a thermalinsulator having a high thermal resistance. In the exemplary embodiment,the mold tool is heated prior to the injection step 54, to provideisothermic conditions. Various techniques may be used to heat the mold,as will be recognized by those skilled in the art. A temperature sensormay be placed in the mold cavity, and temperature of the thermoplasticflow, the mold tool, and/or the build environment can then be adjustedas needed to maintain isothermic conditions. Maintaining the temperatureof the mold tool at approximately the extrusion temperature of theproduction thermoplastic prevents premature solidification of theproduction thermoplastic, so that the mold cavity can fill completelybefore the prototype part hardens. It should be understood, however,that some materials and process parameters may provide isothermicconditions without the need for pre-heating the mold, in which case step52 may be omitted. Also, it may be desirable to heat the mold cavity andinner mold surface, rather than heating the entire mold.

During (and prior to) the injection step 55, a vacuum may be drawn uponthe mold tool. This may be done, for example, by placing the mold toolin a vacuum chamber. The vacuum will remove gases from the mold tool andthe sprue, through the porosity of the mold or the vent. The vacuum willassist in pulling the injected material into the mold cavity. The vacuumwill also facilitate filling the mold cavity more fully and consistentlythan would be achieved under normal atmospheric conditions, resulting ina void-free part.

When the mold cavity is filled, injection of material into the moldcavity is terminated and the prototype part is allowed to cool insidethe mold tool, in a step 56. Cooling may take from 1 to 2 hours.Pressure may be placed on the mold tool during cooling, to compensatefor shrinkage of the mold tool and the prototype part. If desired,active cooling may be used to speed the cooling process. Also,monitoring of pressure in the mold cavity may be continued duringcooling. Cooling in the mold tool may be allowed to continue until theprototype part reaches room temperature, or the part may be removed whenit is substantially cool.

Using the method of the present invention, a prototype plastic injectionmolded part can be produced within a 24-hour time period. Up to 50prototype injection molded parts could be produced within 48 hours.

It should be understood that a mold tool for use in carrying out thepresent invention need not be limited to a thermoplastic mold build by adeposition modeling process. Rather, any non-conductive plastic moldtool compatible with the injection process may be utilized, including,for example, mold tools formed by stereolithography, thermoset moldtools, and mold tools formed by a machine-removable process (e.g.,CAM/CNC). A plastic material forming the mold tool may include variousfillers, additives and the like.

Although the present invention has been described with reference topreferred embodiments, the invention is defined by the claims. Workersskilled in the art will recognize that changes may be made in form anddetail without departing from the spirit and scope of the invention.

For example, in one alternate embodiment, the mold tool is transparentand a photopolymer is used to form the prototype part. The photopolymeris injected into the mold cavity and cured by exposure to light. Inanother embodiment, thermoset reaction injection molding techniques areemployed. Two or more reactant materials are mixed together to form athermoset resin, which is injected into the mold tool. The resin iscured to form the prototype part by applying heat to the mold tool.

1. A method for making a prototype plastic injection molded part,comprising the steps of: providing a plastic mold tool defining a moldcavity; injecting a liquified ribbon of plastic material into the moldcavity using an extrusion head at a pressure of less than 20 psi, untilthe material fills the cavity; curing the plastic material in the moldcavity to form the prototype part.
 2. The method of claim 1, wherein theplastic mold tool is formed from an alkali-soluble modeling material. 3.The method of claim 1 and further comprising: heating the mold tool toapproximately the extrusion temperature, prior to the injecting step. 4.The method of claim 1, further comprising building the plastic mold toolusing a rapid prototyping technique based at least in part on computerdata.
 5. The method of claim 4, wherein the plastic mold tool is builtin a fused deposition modeling machine.
 6. The method of claim 5,wherein the building step and the injecting step are performed in thesame fused deposition modeling machine.
 7. The method of claim 1, andfurther comprising the step of: clamping the mold tool to a fixture witha clamping force of less than or equal to 10 tons, prior to theinjecting step.
 8. The method of claim 1, wherein the injection step isan adiabatic process.
 9. A method for making a prototype plasticinjection molded part, comprising the steps of: providing a plastic moldtool defining a mold cavity; injecting a thermoplastic material into themold cavity as a liquified ribbon of material using an extrusion head,at a pressure of less than 20 psi, so that the thermoplastic materialfills the mold cavity; and cooling the thermoplastic material in themold cavity to form the prototype part.
 10. The method of claim 9 andfurther comprising the step of: building the mold tool using a rapidprototyping technique, based on computer file data representing adesired prototype part.
 11. The method of claim 10, wherein the moldtool is built in a fused deposition modeling machine.
 12. The method ofclaim 11, wherein the building step and the injecting step are performedin the same fused deposition modeling machine.
 13. The method of claim9, wherein the injecting step is done in a fused deposition modelingmachine.
 14. The method of claim 9, wherein the mold material isinjected into the mold cavity using a melt extruder.
 15. The method ofclaim 14, wherein the melt extruder comprises a filament pump.
 16. Themethod of claim 14, wherein the melt extruder comprises a piston pump.17. The method of claim 14, and further comprising the steps of:positioning a sprue in the mold tool such that a dispensing end of thesprue is directed into the mold cavity; and attaching an inlet end ofthe sprue to a dispensing tip of the melt extruder; wherein thethermoplastic material is injected from the melt extruder into the moldcavity via the sprue.
 18. The method of claim 14, wherein ultrasonicenergy is induced in the extruder during the injecting step creating athixotropic flow of thermoplastic.
 19. The method of claim 9 and furthercomprising: heating the mold tool to approximately the extrusiontemperature, prior to the injecting step.
 20. The method of claim 9 andfurther comprising: coating surfaces of the mold cavity with a releaseagent, prior to the injecting step.
 21. The method of claim 9, andfurther comprising the step of: monitoring pressure in the mold cavityduring the injecting step and responsively adjusting the injectionpressure.
 22. The method of claim 9, wherein the plastic mold tool isformed from an alkali-soluble modeling material.
 23. The method of claim9, wherein the thermoplastic material comprises a curable material. 24.The method of claim 9, wherein the curable material is selected from thegroup consisting of a photopolymer and a thermosetting material.
 25. Themethod of claim 9, and further comprising the step of: clamping the moldtool to a fixture with a clamping force of less than or equal to 10tons, prior to the injecting step.
 26. The method of claim 9, andfurther comprising the step of: maintaining constant pressure on themold tool during the cooling step to compensate for shrinkage of theprototype part and the mold tool.
 27. The method of claim 9, wherein theprototype part is cooled in the mold cavity to a temperatureapproximating room temperature.
 28. The method of claim 9, and furthercomprising the step of: vapor smoothing surfaces of the mold tool priorto the injecting step.
 29. The method of claim 9, wherein thethermoplastic material is selected from a group consisting of ABS,polycarbonate, polystyrene, acrylics, amorphous polyamides, polyesters,polyphenylsulfone, polyphenylene ether, nylon, PEEK, PEAK, and blendsthereof.
 30. The method of claim 9, wherein the plastic mold tool isformed from a thermoplastic material comprising at least 50 weightpercent of a thermoplastic selected from the group consisting ofpolycarbonate, polystyrene, acrylics, amorphous polyamides, polyesters,polyphenylsulfone, polysulfone, polyphenylene ether, nylon, PEEK, PEAK,poly(2-ethyl-2-oxazoline), and blends thereof.
 31. The method of claim30, wherein the thermoplastic forming the mold tool comprises apolyphenylsulfone-based resin.
 32. The method of claim 30, wherein thethermoplastic forming the mold tool comprises a polyphenylsulfone-basedresin and the injected thermoplastic is ABS.
 33. The method of claim 9,wherein the injecting step is performed using a vacuum assist.
 34. Themethod of claim 9, wherein the injection step is an adiabatic process.35. A method for making a prototype plastic injection molded part,comprising the steps of: providing a plastic mold tool defining a moldcavity, the plastic mold tool being built with an additive process rapidprototyping machine; injecting a polymer into the mold cavity using theadditive process rapid prototyping machine at a pressure of less than 20psi, until the material fills the cavity; and solidifying the polymer inthe mold cavity, thereby forming the molded prototype part.
 36. Themethod of claim 35, wherein the polymer comprises a photopolymer, andwherein solidifying the polymer comprises exposing the polymer to light.37. The method of claim 35, wherein the rapid prototyping machinecomprises a fused deposition modeling machine.
 38. The method of claim35, and further comprising vapor smoothing the plastic mold tool priorto the injecting step.
 39. The method of claim 35, and furthercomprising monitoring pressure in the mold cavity during the injectingstep and responsively adjusting the injection pressure.
 40. The methodof claim 35, wherein the injection step is an adiabatic process.
 41. Amethod for making a prototype plastic injection molded part, comprisingthe steps of: providing a plastic mold tool defining a mold cavity;providing a supply of two or more reactant materials which form athermoset resin when reacted together; mixing the reactant materialstogether; injecting the reactant materials from an extruder into themold cavity as a liquified ribbon of material, at a controlled pressureof less than 20 psi, so that the reactant materials fill the cavity;heating the reactant materials in the mold cavity to form the moldedprototype part; and cooling the molded prototype part in the moldcavity.
 42. The method of claim 41, further comprising building theplastic mold tool using a rapid prototyping technique based at least inpart on computer data.
 43. The method of claim 42, wherein the plasticmold tool is built in a fused deposition modeling machine.
 44. Themethod of claim 41, wherein the injection step is an adiabatic process.